1. Field of the Invention
This invention relates to medical devices and methods. More specifically, the invention relates to a prosthetic disc for intervertebral insertion, such as in the lumbar and cervical spine.
In the event of damage to a lumbar or cervical intervertebral disc, one possible surgical treatment is to replace the damaged disc with a disc prosthesis. Several types of intervertebral disc prostheses are currently available. For example, one type of intervertebral disc prosthesis is provided by Waldemar Link GmbH & Co under the trademark LINK® SB Charite. This prosthesis includes upper and lower prosthesis plates or shells which locate against and engage the adjacent vertebral bodies, and a low friction core between the plates. The core has upper and lower convexly curved surfaces and the plates have corresponding, concavely curved recesses which cooperate with the curved surfaces of the core. This allows the plates to slide over the core to allow required spinal movements to take place. The curved recesses in the plates are surrounded by annular ridges which locate, at the limit of sliding movement of the plates over the core, in opposing upwardly and downwardly facing, peripheral channels surrounding the curved surfaces of the core.
This type of disc configuration is described in EP 1142544A1 and EP 1250898A1, assigned to Waldemar Link GmbH & Co. A drawback of such configurations is that the provision of the peripheral ribs and channels limits the areas available for bearing and sliding contact between the plates and core, and accordingly the loads which can be transmitted by the prosthesis. As a result of the relatively small bearing areas, it is believed that at least the core will be subject to rapid wear and have a relatively short lifespan. Also, because the core is in effect merely “clamped” between the plates, this configuration does not allow for secure retention of the core. In one alternative arrangement, the curved surfaces of the core carry opposing, elongate keys that locate in elongate grooves in the plates and another alternative arrangement in which the plates have opposing elongate keys that locate in elongate grooves in the opposite curved surfaces of the core. These key and groove arrangements allow the plates to slide over the core within the limits of the length of the grooves, in one direction only. Although allowance is made for some lateral play of the keys in the grooves, very little sliding movement of the plates over the core can take place in the orthogonal vertical plane, and this is considered to be a serious drawback of this design.
Other currently available intervertebral disc prostheses have similar and/or other drawbacks. Typically, drawbacks include insufficient resistance to wear and tear, restricted range of motion and/or insufficient ability of the prosthesis to adhere to vertebral bone.
Therefore, a need exists for improved intervertebral disc prostheses. Ideally, such improved prostheses would resist wear and tear, provide a desired range of motion and adhere well to vertebral bone. At least some of these objectives will be met by the present invention.
2. Description of the Background Art
Published U.S. patent applications 2002/0035400A1 and 2002/0128715A1 describe disc implants which comprise opposing plates with a core between them over which the plates can slide. The core receives one or more central posts, which are carried by the plates and which locate in opposite ends of a central opening in the core. Such arrangements limit the load bearing area available between the plates and core.
Other patents related to intervertebral disc prostheses include U.S. Pat. Nos. 4,759,766; 4,863,477; 4,997,432; 5,035,716; 5,071,437; 5,370,697; 5,401,269; 5,507,816; 5,534,030; 5,556,431; 5,674,296; 5,676,702; 5,702,450; 5,824,094; 5,865,846; 5,989,291; 6,001,130; 6,022,376; 6,039,763; 6,139,579; 6,156,067; 6,162,252; 6,315,797; 6,348,071; 6,368,350; 6,416,551; 6,592,624; 6,607,558 and 6,706,068. Other patent applications related to intervertebral disc prostheses include U.S. patent application Publication Nos.: 2003/0009224; 2003/0074076; 2003/0191536; 2003/0208271; 2003/0135277; 2003/0199982; 2001/0016773 and 2003/0100951. Other related patents include WO 01/01893A1, EP 1344507, EP 1344506, EP 1250898, EP 1306064, EP 1344508, EP 1344493, EP 1417940, EP 1142544, and EP 0333990.